Fix pyarrow.types & gandiva cython bindings

[asfimport]

1. 'types.py' didn't export _as_type, causing failures in certain cython/python combinations. I am surprised to see that the CI didn't fail.
2. After updating the gandiva cpp part (ARROW-3587), the cython bindings (ARROW-3602) are not consistent.

Reporter: Siyuan Zhuang / @suquark
Assignee: Siyuan Zhuang / @suquark

PRs and other links:

• GitHub Pull Request #2931

_{Note: This issue was originally created as ARROW-3742. Please see the migration documentation for further details.}

[asfimport]

Philipp Moritz / @pcmoritz:
Issue resolved by pull request 2931
#2931